GAE EntityManager find throwing null

我一直在玩GAE Java後端，我試圖用實體管理器來更新給定一個鍵的實體。首先，我需要在實體管理器中使用.find方法查找並從數據存儲中獲取實體。但即使我知道實體存在（我在數據存儲查看器中看到它），.find方法仍然會拋出空指針錯誤。有人知道什麼是錯的嗎？謝謝。GAE EntityManager find throwing null

實體被定義爲：

@PersistenceCapable(detachable = "true") 
public class Event { 
@PrimaryKey 
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY) 
private Key key; 
private String name; 
private String description;

這裏就是代碼失敗：

@ApiMethod(name = "updateEvent", path="update_event") 
public Event updateEvent(Event event) { 
    EntityManager mgr = getEntityManager(); 
    if(mgr==null || event==null || event.getKey()==null) 
     return null; 
    try { 
     Event item = mgr.find(Event.class, event.getKey()); // java.lang.NullPointerException thrown here

這裏的堆棧跟蹤：

com.clearscoreboard.EventEndpoint updateEvent: Stacktrace: java.lang.NullPointerException 
at com.google.appengine.api.datastore.KeyTranslator.convertToPb(KeyTranslator.java:49) 
at com.google.appengine.api.datastore.AsyncDatastoreServiceImpl$V3KeyBatcher.toPb(AsyncDatastoreServiceImpl.java:111) 
at com.google.appengine.api.datastore.AsyncDatastoreServiceImpl$V3KeyBatcher.toPb(AsyncDatastoreServiceImpl.java:102) 
at com.google.appengine.api.datastore.Batcher$BatchIterator.<init>(Batcher.java:176) 
at com.google.appengine.api.datastore.Batcher$1.<init>(Batcher.java:270) 
at com.google.appengine.api.datastore.Batcher.getBatches(Batcher.java:270) 
at com.google.appengine.api.datastore.AsyncDatastoreServiceImpl.doBatchGet(AsyncDatastoreServiceImpl.java:301) 
at com.google.appengine.api.datastore.AsyncDatastoreServiceImpl.get(AsyncDatastoreServiceImpl.java:280) 
at com.google.appengine.api.datastore.DatastoreServiceImpl$1.runInternal(DatastoreServiceImpl.java:69) 
at com.google.appengine.api.datastore.DatastoreServiceImpl$1.runInternal(DatastoreServiceImpl.java:66) 
at com.google.appengine.api.datastore.TransactionRunner.runInTransaction(TransactionRunner.java:29) 
at com.google.appengine.api.datastore.DatastoreServiceImpl.get(DatastoreServiceImpl.java:66) 
at com.google.appengine.api.datastore.DatastoreServiceImpl.get(DatastoreServiceImpl.java:56) 
at com.google.appengine.datanucleus.WrappedDatastoreService.get(WrappedDatastoreService.java:60) 
at com.google.appengine.datanucleus.EntityUtils.getEntityFromDatastore(EntityUtils.java:665) 
at com.google.appengine.datanucleus.DatastorePersistenceHandler.fetchObject(DatastorePersistenceHandler.java:543) 
at org.datanucleus.state.JDOStateManager.loadFieldsFromDatastore(JDOStateManager.java:1638) 
at org.datanucleus.state.JDOStateManager.validate(JDOStateManager.java:3511) 
at org.datanucleus.ObjectManagerImpl.findObject(ObjectManagerImpl.java:3379) 
at org.datanucleus.api.jpa.JPAEntityManager.find(JPAEntityManager.java:358) 
at org.datanucleus.api.jpa.JPAEntityManager.find(JPAEntityManager.java:256) 
at com.clearscoreboard.EventEndpoint.updateEvent(EventEndpoint.java:240) 
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57) 
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) 
at java.lang.reflect.Method.invoke(Method.java:606) 
at com.google.apphosting.runtime.security.shared.intercept.java.lang.reflect.Method_$1.run(Method_.java:179) 
at java.security.AccessController.doPrivileged(Native Method) 
at com.google.apphosting.runtime.security.shared.intercept.java.lang.reflect.Method_.privilegedInvoke(Method_.java:176) 
at com.google.apphosting.runtime.security.shared.intercept.java.lang.reflect.Method_.invoke_(Method_.java:137) 
at com.google.apphosting.runtime.security.shared.intercept.java.lang.reflect.Method_.invoke(Method_.java:45) 
at com.google.api.server.spi.SystemService.invokeServiceMethod(SystemService.java:359) 
at com.google.api.server.spi.SystemServiceServlet.execute(SystemServiceServlet.java:124) 
at com.google.api.server.spi.SystemServiceServlet.doPost(SystemServiceServlet.java:82) 
at javax.servlet.http.HttpServlet.service(HttpServlet.java:637) 
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) 
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511) 
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166) 
at com.google.apphosting.utils.servlet.ParseBlobUploadFilter.doFilter(ParseBlobUploadFilter.java:125) 
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
at com.google.apphosting.runtime.jetty.SaveSessionFilter.doFilter(SaveSessionFilter.java:35) 
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
at com.google.apphosting.utils.servlet.JdbcMySqlConnectionCleanupFilter.doFilter(JdbcMySqlConnectionCleanupFilter.java:60) 
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43) 
at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157) 
at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388) 
at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216) 
at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182) 
at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765) 
at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418) 
at com.google.apphosting.runtime.jetty.AppVersionHandlerMap.handle(AppVersionHandlerMap.java:266) 
at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152) 
at org.mortbay.jetty.Server.handle(Server.java:326) 
at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542) 
at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923) 
at com.google.apphosting.runtime.jetty.RpcRequestParser.parseAvailable(RpcRequestParser.java:76) 
at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404) 
at com.google.apphosting.runtime.jetty.JettyServletEngineAdapter.serviceRequest(JettyServletEngineAdapter.java:146) 
at com.google.apphosting.runtime.JavaRuntime$RequestRunnable.run(JavaRuntime.java:446) 
at com.google.tracing.TraceContext$TraceContextRunnable.runInContext(TraceContext.java:437) 
at com.google.tracing.TraceContext$TraceContextRunnable$1.run(TraceContext.java:444) 
at com.google.tracing.CurrentContext.runInContext(CurrentContext.java:188) 
at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContextNoUnref(TraceContext.java:308) 
at com.google.tracing.TraceContext$AbstractTraceContextCallback.runInInheritedContext(TraceContext.java:300) 
at com.google.tracing.TraceContext$TraceContextRunnable.run(TraceContext.java:441) 
at com.google.apphosting.runtime.ThreadGroupPool$PoolEntry.run(ThreadGroupPool.java:251) 
at java.lang.Thread.run(Thread.java:724)

來源

2014-03-04 Ryan

您可以提供您構建Key對象的方式嗎？ –

檢查getEntityManager方法。如果沒有結果嘗試這樣的事情（100％工作）：

public T get(Key key) throws Exception { 
    EntityManager mgr = EMF.get().createEntityManager(); 
    T entity = null; 
    try { 
     entity = mgr.find(entityClass, key); 
    } finally { 
     mgr.close(); 
    } 
    if (entity == null) { 
     throw DS_NOT_FOUND(); 
    } 
    return entity; 
}

和：

public final class EMF { 
private static final EntityManagerFactory emfInstance = Persistence.createEntityManagerFactory("transactions-optional"); 

private EMF() { 
} 

public static EntityManagerFactory get() { 
    return emfInstance; 
}

肯定這會工作。

來源

2014-06-19 16:57:58 Alex91

GAE EntityManager find throwing null

回答

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